1. Beamforming
The next generation mobile wireless communication system, which is referred to as “5G,” will support a diverse set of use cases and a diverse set of deployment scenarios. 5G will encompass an evolution of today's 4G LTE (Long Term Evolution) networks and the addition of a new, globally standardized radio-access technology known as “New Radio” (NR).
The diverse set of deployment scenarios includes deployment at both low frequencies (100s of MHz), similar to LTE today, and very high frequencies (mm waves in the tens of GHz). At high frequencies, propagation characteristics make achieving good coverage challenging. One solution to the coverage issue is to employ beamforming (e.g., high-gain beamforming) to achieve satisfactory link budget.
Beamforming is an important technology in future radio communication systems. It can improve performance both by increasing the received signal strength, thereby improving the coverage, and by reducing unwanted interference, thereby improving the capacity. Beamforming can be applied both in a transmitter and a receiver.
In a transmitter, beamforming involves configuring the transmitter to transmit the signal in a specific direction (“beam”), or in two or more directions, and not in other directions. In a receiver, beamforming involves configuring the receiver to receive signals from a certain direction (or a few directions) and not from other directions. When beamforming is applied in both the transmitter and the receiver for a given communication link, the combination of the beam used by the transmitter to transmit a signal to the receiver and the beam used by the receiver to receive the signal is referred to as a beam-pair link (BPL). Generally, the beamforming gains are related to the widths of the used beams: a relatively narrow beam provides more gain than a wider beam. A BPL can be defined for DL and UL separately or jointly based on reciprocity assumptions.
For a more specific description of beamforming, one typically talks about beamforming weights rather than beams. On the transmission side, the signal to be transmitted is multiplied with beamforming weights (e.g., complex constants) before being distributed to the individual antenna elements. There are separate beamforming weights for each antenna element, which allows maximum freedom in shaping the transmission beam given the fixed antenna array. Correspondingly, on the receiving side, the received signal from each antenna element is multiplied separately with the beamforming weights before the signals are combined. However, in the context of the present text, the description is easier to follow if the somewhat simplified notion of beams, pointing in certain physical directions, is adopted.
Beamforming is a mature subject today. This section just aims at presenting the basics. Referring now to FIG. 1, FIG. 1 shows an idealized one-dimensional beamforming case. In case it is assumed that a wireless communication device (WCD) (e.g., a user equipment (UE), such as a smartphone, laptop, tablet, phablet etc.; a machine-type communication device, such as a smart appliance, a sensor, etc.; or other device capable of wireless communication) is located far away from the antenna array it follows that the difference in travel distance from the base station to the WCD, between adjacent antenna elements, is l=kλ sin(θ), where kλ is the antenna element separation. Here k is the separation factor which may be 0.5-0.7 in a typical correlated antenna element arrangement. This means that if a reference signal siejωt transmitted from the i:th antenna element will arrive at the WCD antenna as a weighted sum
      s    UE    =                    ∑                  i          =          0                          N          -          1                    ⁢                        s          i                ⁢                  h          i                ⁢                  e                      j            ⁢                                                  ⁢                          ω              ⁡                              (                                  t                  -                                      il                    c                                                  )                                                          =                            e                      j            ⁢                                                  ⁢            ω            ⁢                                                  ⁢            t                          ⁢                              ∑                          i              =              1                                      N              -              1                                ⁢                                    s              i                        ⁢                          h              i                        ⁢                          e                                                -                  j                                ⁢                                                      ik                    ⁢                                                                                  ⁢                                                                  λ                        ⁢                        sin                                            ⁡                                              (                        θ                        )                                                                                                                        f                      c                                        ⁢                    λ                                                                                          =                        e                      j            ⁢                                                  ⁢            ω            ⁢                                                  ⁢            t                          ⁢                              ∑                          i              =              1                                      N              -              1                                ⁢                                    s              i                        ⁢                          h              i                        ⁢                                          e                                                      -                    j                                    ⁢                                                            ik                      ⁢                                                                                          ⁢                                              sin                        ⁡                                                  (                          θ                          )                                                                                                            f                      c                                                                                  .                                          
Here ω is the angular carrier frequency, hi is the complex channel from the i:th antenna element, t is the time, and fc is the carrier frequency. In the above equation θ and hi are unknown. In case of a feedback solution, the WCD therefore needs to search for all complex channel coefficients hi and the unknown angle θ. For this reason the standard defines a codebook of beams in different directions given by steering vector coefficients like wm,i=e−jf(m,i), where m indicates a directional codebook entry. The WCD then tests each codebook and estimates the channel coefficients. The information rate achieved for each codebook entry m is computed and the best one defines the direction and channel coefficients. This is possible since si is known. The result is encoded and reported back to the base station. This provides the base station with a best direction (codebook entry) and information that allows it to build up a channel matrix H. This matrix represents the channel from each of the transmit antenna elements to each of the receive antenna elements. Typically, each element of H is represented by a complex number.
The channel matrix can then be used for beamforming computations, or the direction represented by the reported codebook entry can be used directly. In case of MIMO transmission the MIMO beamforming weight matrix W needs to be determined so that a best match to the requirement WH=I is achieved where I denotes the identity matrix. In case of an exact match each layer will become independent of other layers. This concept can be applied for single users or multiple users.
When reciprocity is used the channel coefficients can, in principle, be directly estimated by the base station from WCD uplink transmission. So called sounding reference signals, SRS, are used for this purpose. The estimated channel is then used to compute the combining weight matrix according to some selected principle, and then used for downlink transmission. This works since the uplink and downlink channels are essentially the same when reciprocity is applicable.
2. 5G 3GPP Reference Signals Supporting Beamforming
Some of the description herein is given in terms of the 3GPP terminology for the 4G LTE system, since the standardization of the 5G counterparts is not yet finalized. The operation of the 5G functionality is expected to be essentially the same as in the 4G system.
The channel state information reference signals, CSI-RS, which has been available since release 11, are assigned to a specific antenna port. These reference signals may be transmitted to the whole cell or may be beamformed in a WCD specific manner. In 3GPP from release 13 two classes of CSI-RS reporting mode has been introduced: class A CSI-RS refers to the use of fixed-beam codebook based beamforming, while a class B CSI-RS process may send beamformed CSI-RS in any manner.
A CSI-RS process in a WCD comprises detection of selected CSI-RS signals, measuring interference and noise on a CSI Interference Measurement (CSI-IM) resource, and reporting of the related CSI information, in terms of CQI, RI and PMI. Here CSI denotes channel state information, CQI denotes channel quality indication, RI denotes (channel matrix) rank indications and PMI denotes pre-coder matrix index, i.e. the selected codebook entry. A WCD may report more than one set of CQI, RI and PMI, i.e. information for more than one codebook entry. Up to 4 CSI-RS processes can be set up for each WCD, starting in 3GPP release 11.
3. 5G 2D Codebooks and Antenna Port Relations
As stated above the codebook of the 3GPP standard is defined to represent certain directions. In release 13, directions in both azimuth and elevation is defined, thereby allowing 2D beamforming to be used. These 4G codebooks are specified in detail in 3GPP TR 36.897. A similar definition, but with finer granularity is expected for the 3GPP 5G standard.
In order to illustrate that the codebooks indeed define specific directions, it can be noted that the formula for the azimuth codebook is
            w      k        =                            1                      K                          ⁢                  exp          (                                    -              j                        ⁢                                          2                ⁢                π                            λ                        ⁢                          (                              k                -                1                            )                        ⁢                          d              V                        ⁢            cos            ⁢                                                  ⁢                          θ              etilt                                )                ⁢                                  ⁢        for        ⁢                                  ⁢        k            =      1        ,  …  ⁢          ,      K    .  
It has the same structure as discussed above. Similarly, the vertical codebook in that document is given by
            v              l        ,        i              =                            1                      L                          ⁢                  exp          (                                    -              j                        ⁢                                          2                ⁢                π                            λ                        ⁢                          (                              l                -                1                            )                        ⁢                          d              H                        ⁢            sin            ⁢                                                  ⁢                          ϑ              i                                )                ⁢                                  ⁢        for        ⁢                                  ⁢        l            =      1        ,  …  ⁢          ,      L    .  
In the two above equations it is only the structure that is needed here, the details of the involved quantities is of less importance and is not reproduced here, see 3GPP TR 36.897 for all details. Finally, it is noted that a 2D beam is obtained by a multiplication of the two above equations.
4. Interference Avoidance with RAIT
Reciprocity-assisted interference aware transmission (RAIT) is a technique that is applicable primarily for Time Division Duplex (TDD) deployments, where channel reciprocity can be used. Briefly, RAIT offers a unified approach to single point techniques like MU-MIMO and beamforming, and to multi-point techniques like CoMP and D-MIMO. The key to achieve this is availability of a high fidelity multi-antenna element spatial 1D or 2D matrix channel estimate H. Since reciprocity is normally assumed to hold for RAIT, the channel matrix can be estimated by application of e.g. sounding reference signals (SRS) in the uplink. The channel matrix is then also valid for downlink transmission. By formulation of a criterion that embeds the above techniques as special cases, a combining weight matrix W can then be computed and used to steer the downlink transmit power in an optimal way. One particular feature of RAIT is that it is capable to avoid transmission in directions where interference is likely to be created, between users.